Hall Effect sensors are among the most widely used magnetic sensors. Hall Effect sensors incorporate a Hall Effect plate, which is either an n- or p-doped area, supplied with bias current/voltage. In presence of a magnetic field the carriers that are moving in the doped area are deflected by the Lorentz force, and a Hall electrical field appears. The Hall voltage (Vh) appears across the positive and negative contacts of the Hall effect plate. Front-end circuitry provided with the sensor converts the Hall voltage to a data indicative of the sensed magnetic field.
An ideal Hall element when biased with a current thus generates a Hall voltage that is proportional to the product of the bias current and the applied magnetic field. In practice, fabrication imperfections and environmental conditions such as temperature and stress give rise to an additional voltage referred to as an “offset voltage” (Vos). The presence of the Vos compromises the accuracy of the magnetic field measurement obtained by the Hall element.
One technique which has been developed to reduce the effects of the Vos is a “spinning current” technique. This technique involves sequentially biasing a Hall element through its different bias states. By repeating the sequential biasing indefinitely, the magnetic field dependent voltage is modulated to a high frequency while the Vos is established as a DC value. Alternatively, the Vos is modulated to a high frequency while the magnetic field dependent voltage is established as a DC value. In either event, because the two voltages are separated by frequency, the desired signal may be recovered in either the analog or digital domain by demodulation and filtering.
While the spinning current technique is generally effective, some limitations are encountered. For example, the magnitude of the Vos is several orders of magnitude larger than the maximum magnitude of the desired signal. Consequently, current spinning results in a small AC signal, representing the desired signal, riding on top of a large DC signal or offset.
The large DC offset places a heavy burden on the front end amplifier used to amplify the combined voltages and the analog-to-digital converter (ADC), the circuits incorporating digital demodulation, since the front end amplifier and ADC must accommodate both the small modulated signal of interest as well as the large Vos. By way of example, in order to prevent saturation of a circuit configured to measure magnetic fields of about 60 dB of dynamic range, the circuit may require a design that can process up to 120 dB of linear dynamic range. The additional 60 dB of dynamic range presents a significant penalty in cost and power dissipation. Power consumption is a particular concern with the evolving market of portable low power devices including cell phones.
A Hall effect sensor circuit that reduces sensor offset would be beneficial. A Hall effect circuit which reduces sensor offset while reducing the required dynamic range of front end and backend circuitry would be further beneficial. A Hall sensor which reduces power dissipation and circuit complexity would also be beneficial.